Pneumatic tire and method of manufacturing tire vulcanization mold

ABSTRACT

A pneumatic tire has an outer surface with an arithmetical mean roughness Ra in the range of 0.1 μm to 0.6 μm when a reference length Lr is 20 μm, wherein the outer surface has an irregularity diameter in the range of 40-230 μm and an irregularity depth in the range of 3-22 μm. A method of manufacturing a tire vulcanization mold includes propelling spherical abrasive grains onto the surface of the mold under the condition that a propelling pressure is in the range of 0.1-0.6 MPa, so as to process the surface of the mold.

This is a divisional application of application Ser. No. 13/637,585filed Oct. 19, 2012, which in turn is a U.S. National Stage ofPCT/JP2011/058377 filed Mar. 25, 2011, which claims foreign priority toJP 2010-073477 filed Mar. 26, 2010. The disclosure of the priorapplications is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a pneumatic tire and a method ofprocessing the surface of a tire vulcanization mold. In particular, thepresent invention relates to a pneumatic tire that has an appropriategloss and a deeply-black and moist texture on its surface, therebyachieving a good appearance, and to a method of manufacturing a tirevulcanization mold that is capable of forming such a pneumatic tire.

BACKGROUND ART

In general, a tire vulcanization mold is firstly made by casting, andthen is finished by subjecting its surface to an abrasive blastingprocess using abrasive grains made of alumina or the like.

On the surface of the mold that has been subjected to an abrasiveblasting process, irregularities of random diameters and depths areformed. Once these random irregularities are transferred to the surfaceof the vulcanized tire, the tire surface disorderly diffuses light, andthereby the tire ends up having no gloss while having a black and yetcoarse texture.

Herein, the term “no gloss” refers to the condition in which the valuemeasured by a gloss meter is in the range of 0.1 to less than 1. A tirehaving no gloss often has a coarse texture on its surface.

In this regard, for example, Patent Document 1 (JP 2000-142026 A)discloses a pneumatic tire wherein the surface roughness (RaT) of themolded tire is in the range of 1.5-20 μm when it is measured by astylus-type surface roughness tester. Patent Document 1 describes thatthe surface gloss of the molded tire may be enhanced and the appearanceof the tire may be improved.

Patent Document 2 (JP 2004-017964 A) discloses a tire mold having aradially inner surface for forming a tread and a pair of sidewalls, thetire mold comprising: at least one sidewall forming surface having asurface finish of less than 0.381 microns, There is described that ahighly light reflective surface on a tire sidewall is achieved in anovel way. The resultant tire has highly light reflective surfacesachieved in a way that is clearly recognizable regardless of theobserver's relative position.

The art disclosed in aforementioned Patent Documents 1 and 2 enhancesthe gloss of the tire surface and enhances the reflectance of the tiresurface, by controlling the surface roughness Ra, thereby improving theappearance of the tire.

The inventors manufactured tires using two types of molds that weresubjected to different surface processes and examined the surfaceroughness Ra and gloss of the tires. They found out that the tires withsubstantially equal surface roughness Ra did not always have the samemeasurement gloss value. The evaluation results are shown in Table 1.

In Table 1, Process A indicates an abrasive blasting process performedon a surface of a tire mold using alumina abrasive grains, and Process Bindicates an abrasive blasting process performed on a surface of a tiremold using spherical abrasive grains.

TABLE 1 Processing method Process A Process B Surface roughness Ra 3.72μm 3.42 μm Tire gloss 0.4 2.7

According to Table 1, out of two types of surface processing methods,i.e. Process A and Process B, by which tires of substantially equalsurface roughness Ra are obtained, Process B achieves the tire gloss of2.7, which is good, while Process A achieves the tire gloss of 0.4,which is insufficient.

According to the foregoing results, in order to control the tire glossso as to constantly achieve a tire with the gloss of not less than 1, itis not sufficient to merely maintain the surface roughness Ra of thetire at a constant value.

Patent Document 3 (JP 2003-300214 A) discloses a tire vulcanizing moldhaving a molding surface for vulcanizing a tire, wherein at least a partof the molding surface is provided with a rough-surface molding surface,which is configured such that its ten points mean roughness Rz is 5-100μm and it has the surface roughness such that its average distancebetween peaks in a local area S is 20-150 μm. Patent Document 3describes that, since the mold has the rough-surface molding surfacewhich has limited surface roughness, it is possible to form a pneumatictire comprising, on its outer surface, a rough-surface portion havingsubstantially the same surface roughness as the rough-surface moldingsurface. Such a rough-surface portion can diffusely reflect extraneouslight to a preferable extent, thereby enabling the tire to appear deeplyblack. Accordingly, additives seeping through the tire etc. may be lessvisible and the appearance of the tire may be improved over a longperiod of time.

The inventors manufactured tires using the tire vulcanizing mold havingthe structure of the invention disclosed in aforementioned PatentDocument 3 and evaluated the surface condition and tire gloss of thetires. They found out that, when such a tire vulcanizing mold was used,tires having no gloss while having a black and yet coarse texture weresometimes manufactured. The evaluation results are shown in Table 2.

TABLE 2 Ten points mean roughness Rz of the mold 13.1 μm Averagedistance between peaks in a local 48.4 μm area S of the mold Conditionof the tire surface black and yet coarse surface texture Tire gloss 0.4

According to the results shown in Table 2, the tire gloss cannot besufficiently controlled by adjusting parameters such as the ten pointsmean roughness Rz and the average distance between peaks in a local areaS of the mold, and thereby the tires sometimes end up having aninsufficient gloss while having a black and yet coarse texture.

Accordingly, it may be concluded that there are other factors than theaforementioned parameters that cause inconsistency in the tire gloss,and it is impossible to constantly obtain tires with an appropriategloss without controlling these factors.

On the contrary, if the gloss of the tire surface is too high, in otherwords if the gloss of the tire surface is not less than 6.0, the tireends up having a whitish, shiny appearance, which is not preferable.

Further, even if the gloss is appropriate, as discussed above, the tiresurface may have a coarse texture, in which case the appearance of thetire is not preferable. In this regard, it may be concluded that thefactor lies in the parameters for roughness of the tire surface.

In general, a preferable appearance of a tire is achieved when the tiresurface has an appropriate gloss and has a deeply-black and moisttexture. However, as discussed above, it is impossible to constantlysupply such a tire using the prior art disclosed in Patent Documents1-3.

Therefore, in order to constantly supply tires with a good appearance,it is required to contemplate a method of manufacturing a tirevulcanization mold in which roughness parameters for the tire surfacethat determine the gloss and texture of the tire surface are adjustedand in which such roughness parameters may be appropriately transferredto the tire.

PATENT DOCUMENTS

Patent Document 1: JP 2000-142026 A

Patent Document 2: JP 2004-017964 A

Patent Document 3: JP 2003-300214 A

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

For the forgoing reasons, the present invention aims to provide apneumatic tire having an appropriate gloss and a deeply-black and moisttexture on its surface, thereby having a good appearance, and to providea method of manufacturing a tire vulcanization mold by which suchpneumatic tires may be constantly manufactured.

Means for Solving the Problem

The inventors contemplated a way to solve the aforementioned problem,and arrived at the present invention based upon the findings that: it iseffective to adjust the surface roughness Ra that is calculated with areference length Lr of a minute value, for controlling the gloss of thetire surface; and by defining the condition of the tire surface from amore macro perspective compared to the surface roughness Ra, it ispossible to provide the tire surface with a deeply-black and moisttexture.

Conventionally, the surface roughness Ra of a tire was typicallymeasured with a reference length Lr in the range of about 800-2500 μm.

The summary of the present invention which solves the aforementionedproblem is as follows. Namely, the present invention provides apneumatic tire that comprises a tread portion and side portions. Atleast one of the tread portion and the side portions has an outersurface having an arithmetical mean roughness Ra in the range of 0.1 μmto 0.6 preferably in the range of 0.381-0.6 μm, when a reference lengthLr is 20 μm. An irregularity diameter of the outer surface is in therange of 40-230 μm. An irregularity depth of the outer surface is in therange of 3-22 μm.

Herein, the term “arithmetical mean roughness Ra” refers to theroughness parameters “an arithmetical mean height Ra” or “anarithmetical mean roughness Ra” defined in Japan Industrial StandardsJIS B 0601 (2001). In this invention, the arithmetic mean roughness Rawas measured using a laser microscope (VK-9710) by Keyence Corporation.

In the present invention, the term “irregularities” on an outer surfaceof a tire refers to the irregularities that are formed when the scarsformed on the surface of a vulcanizing mold by the abrasive material, asshown in FIGS. 6( a)-(c) for example, are transferred onto the outersurface of the tire. The scars are formed by subjecting the surface ofthe mold to an abrasive blasting process using a stream of either finespherical abrasive material, relatively large spherical abrasivematerial or polygonal abrasive material. In the present invention, theterm “an irregularity diameter” refers to the size of the irregularitiesin their diameter direction, and the term “an irregularity depth” refersto the depth of the irregularities.

In the method of manufacturing a tire vulcanization mold according tothe present invention, the mold has a first portion corresponding to atire tread portion, and has second and third portions corresponding totire side portions. Each of the first, second and third portions has asurface, and the method includes propelling spherical abrasive grainsonto at least one of the surfaces of the first, second and thirdportions of the mold under the condition that a propelling pressure isin the range of 0.1-0.6 MPa, so as to process the surface of the mold.

In the aforementioned manufacturing method, it is preferable that theabrasive grains with a grain diameter of not more than 850 μm arepropelled onto the surface of the mold.

It is preferable that the abrasive grains are propelled onto the surfaceof the mold at the propelling pressure of 0,1 MPa to 0.2 MPa,particularly of 0.1 MPa.

EFFECTS OF THE INVENTION

According to the pneumatic tire of the present invention, since thearithmetical mean roughness Ra of an outer surface of at least one of atread portion and side portions, with the reference length Lr of 20 μm,is in the range of 0.1-0.6 μm, it is possible to constantly achieve thetire gloss of not less than 1.

Further, since the irregularity diameter of the tire surface is in therange of 40-230 μm and the irregularity depth of the tire surface is inthe range of 3-22 μm, it is possible to eliminate a coarse texture fromthe tire surface and provide the tire surface with a deeply-black andmoist texture, thereby achieving a good appearance of the tire.

In the method of manufacturing a tire vulcanization mold according tothe present invention, the mold has a first portion corresponding to atire tread portion, and has second and third portions corresponding totire side portions. At least one of the first, second and third portionshas a surface, and the method includes propelling spherical abrasivegrains onto the surface of the mold under the condition that apropelling pressure is in the range of 0.1-0.6 MPa, so as to process thesurface of the mold. According to the tire vulcanization moldmanufactured by this manufacturing method, it is possible to constantlymanufacture tires in which the arithmetical mean roughness Ra of theouter surface of at least one of the tread portion and side portions,when the reference length Lr is 20 μm, is in the range of 0.1-0.6 μm.

In the method of manufacturing a tire vulcanization mold according tothe present invention, if the abrasive grains to be propelled have thegrain diameter of not more than 850 μm, it is possible to manufacture adevice for tire vulcanization that can constantly manufacture tireshaving a more deeply-black and moist texture on the surface of the treadportion or side portions.

Additionally, if the propelling pressure of the abrasive grains is inthe range of 0.1 MPa to 0.2 MPa, more preferably at 0.1 MPa, it ispossible to manufacture a tire vulcanization mold that can manufacturetires with a better appearance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a roughness curve for explaining thearithmetical mean roughness Ra of a tire surface.

FIG. 2 is a sectional view schematically showing the condition of a tiresurface for explaining the relationship between a reference length Lrfor the roughness curve and a reference length Lw for measuring anirregularity diameter and an irregularity depth.

FIG. 3 shows a surface roughness curve of each of the tires used in theexamples of the present invention.

FIG. 4 is a graph showing the relationship between the arithmetical meanroughness Ra of a tire surface with the reference length Lr of 20 μm,and the tire gloss.

FIG. 5 is a diagram showing the ranges of an irregularity diameter andirregularity depth which provide a tire surface in a good condition.

FIGS. 6( a)-6(c) are enlarged views showing examples of irregularitiesof a tire outer surface.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail herein below withreference to the drawings. FIG. 1 shows a roughness curve of a tiresurface when a reference length Lr is 20 μm. In FIG. 1, Z(x) indicatesthe deviation of the roughness curve from a mean line of the roughnesscurve.

The term “arithmetical mean roughness Ra” in a zone extending along thereference length Lr indicates the mean of the absolute value of Z(x) inthe zone, and is calculated by using the following Equation (1):

$\begin{matrix}{{Ra} = {\frac{1}{Lr}{\int_{0}^{Lr}{{{Z(x)}}{x}}}}} & (1)\end{matrix}$

In the pneumatic tire of the present invention, it is required that thearithmetical mean roughness Ra of the outer surface of at least one of atread portion and side portions, that is calculated by using the aboveEquation (1), when the reference length Lr is 20 μm, is in the range of0.1-0.6 μm. Accordingly, the gloss of the tire surface may be not lessthan 1.

FIG. 2 schematically shows the condition of a tire surface from a moremacro point of view. As shown in FIG. 2, aside from the roughness curve,the tire surface is normally formed with irregularities when observedwith a reference length Lw that is longer than the reference length Lr.The texture of the tire surface depends on the diameter and depth of theirregularities.

In the pneumatic tire of the present invention, in which theaforementioned arithmetical mean roughness Ra is in the range of 0.1-0.6μm, it is required that the irregularity diameter is in the range of40-230 μm and the irregularity depth is in the range of 3-22 μm.Accordingly, the tire surface may have a deeply-black and moist texture.

EXAMPLES

A plurality of molds for tire vulcanization, which were subjected todifferent surface processes respectively, were used to manufacturetires. The arithmetical mean roughness Ra with the reference length Lrof 20 μm, the irregularity diameter and the irregularity depth of thesurface of each of the manufactured tires were measured. Further,sensory evaluation was conducted on the condition of the tire surface.The following Tables 4-6 show the evaluation results.

Processes A-H, shown in the following tables, were performed usingdifferent shapes, grain diameters and propelling pressures of theabrasive grains, as shown in Table 3. For the Processes A-H, thepropelling time was 30 seconds, respectively.

TABLE 3 Process A (Conventional) Process B Process C Process D Shape ofabrasive Polygonal Spherical Spherical Spherical grains Grain diameter(μm) 60-80 150-180 300-425 425-600 Propelling pressure 0.6 0.2 0.2 0.2(MPa) Process E Process F Process G Process H Shape of abrasiveSpherical Spherical Spherical Spherical grains Grain diameter (μm)600-850 700-1000 700-1000 700-1000 Propelling pressure 0.2 0.2 0.4 0.1(MPa)

TABLE 4 Process A (Conventional) Process B Process C Process D Tiregloss 0.4-0.5 2.8-3.0 2.3-2.6  2.5-3.0 Ra (μm) 0.73-1.56 0.16-0.300.13-0.24  0.12-0.19 Irregularity  8.74-30.58 45.27-74.26 94.64-142.57121.76-160.37 diameter (μm) Irregularity 5.27-9.83  3.2-9.75 8.31-20.2310.60-14.83 depth (μm) Condition of Black and yet Black and moist Blackand moist Black and moist tire surface coarse (Sensory evaluation)

TABLE 5 Process E Process F Process G Process H Tire gloss 2.7-3.12.9-3.2 2.3-2.5 3.0-4.0 Ra (μm) 0.12-0.33 0.15-0.30 0.26-0.38 0.12-0.19Irregularity 182.14-209.86 194.02-262.33 181.15-355.28 121.76-160.37diameter (μm) Irregularity  12.1-17.71 10.33-23.84 11.52-28.7910.60-14.83 depth (μm) Condition of Black and moist Black and yet Blackand yet Black and moist tire surface coarse coarse (Sensory evaluation)

TABLE 6 Grain diameter of 150- 300- 425- 600- 710- abrasive grains (μm)180 425 600 850 1000 Specific gravity of 2.5 3.85 3.85 3.85 2.5 abrasivegrains Mohs hardness of 6.5 9 9 9 6.5 abrasive grains Propellingdistance (mm) 150 150 150 150 150 Propelling time (s) 30 30 30 30 30Propelling Irregularity 49.0 113.4 136.4 199.7 222.7 pressure diameter(μm) of 0.1 Irregularity 3.7 8.7 10.1 14.7 12.0 MPa depth (μm)Propelling Irregularity 54.0 126.6 150.9 200 253.8 pressure diameter(μm) of 0.2 Irregularity 5.1 13.5 15.7 21.1 21.8 MPa depth (μm)Propelling Irregularity 59.2 144.3 179.4 262.6 310.9 pressure diameter(μm) of 0.4 Irregularity 10.1 21.0 21.6 25.3 34.4 MPa depth (μm)Propelling Irregularity 71.4 177.8 190.2 312.5 354.0 pressure diameter(μm) of 0.6 Irregularity 12.2 31.5 24.7 35.0 39.1 MPa depth (μm)

According to Tables 4 and 5, by using the molds for tire vulcanizationthat have been subjected to Processes B-H, in which the shape of theabrasive grains is spherical, it is possible to obtain tires in whichthe arithmetical mean roughness Ra of the surface, when the referencelength Lr is 20 μm, is not more than 0.6 μm.

Further, by using the molds for tire vulcanization that have beensubjected to Processes B-E, in which the grain diameter of the abrasivegrains is not more than 850 μm, it is possible to obtain tires that havea black and moist texture and a good appearance.

As to the molds for tire vulcanization that were subjected to ProcessesF-H, in which the grain diameter of the abrasive grains was large, being700-1000 μm, in a case where the propelling pressure was high, thesurfaces of the manufactured tires had a coarse texture, while in a caseof Process H, in which the propelling pressure was 0.1 MPa, tires havinga black and moist texture were obtained.

Accordingly, by arranging the shape of the abrasive grains to bespherical, it is possible to manufacture tires having a deeply-black andmoist texture, even when the grain diameter of the abrasive grains islarge, as long as the propelling pressure is low.

According to Table 6, using the molds for tire vulcanization that weresubjected to the surface processes under respective conditions, tires,in which the irregularity diameter of the surface was in the range of40-230 μm and the irregularity depth of the surface was in the range of3-22 μm, were manufactured, except for the case where the grain diameterof the abrasive grains was 600-850 μm and the propelling pressure wasnot less than 0.4 MPa, and the case where the grain diameter of theabrasive grains was 710-1000 μm and the propelling pressure was not lessthan 0.2 MPa.

By controlling the propelling pressure and the grain diameter, a desiredirregularity diameter and irregularity depth may be obtained.

All tires, in which the irregularity diameter and irregularity depthwere within the aforementioned ranges, had a deeply-black and moisttexture on their surfaces and a good appearance.

Tires A-C, which had, on their tire surfaces, different arithmeticalmean roughness Ra′ with the reference length Lr of 800 μm andarithmetical mean roughness Ra with the reference length Lr of 20 μm,were manufactured. Then, the gloss of the tire surfaces and thecondition of the tire surfaces were evaluated. The evaluation resultsshown in the following Table 7 were obtained.

The waveforms of curves of the surface roughness measured with respectto Tires A-C are shown in FIGS. 3( a)-3(c), respectively.

TABLE 7 Tire A Tire B Tire C Method of processing the Process A MirrorProcess B surface of each mold for processing respective tire Waveformof the surface FIG. 3(a) FIG. 3(b) FIG. 3(c) roughness curve of the tiresurface Arithmetical mean roughness 3.72 0.63 3.42 Ra′ when thereference length Lr is 800 μm Arithmetical mean roughness 1.11 0.05 0.30Ra when the reference length Lr is 20 μm Tire gloss 0.4  8.3  2.7 Condition of tire surface Black and yet Whitish and Black and (Sensoryevaluation) coarse shiny moist

According to Table 7, Tire A has a large arithmetical mean roughness Ra,with the reference length Lr of 20 μm, that is 1.11. For this reason,although Tire A has the arithmetical mean roughness Ra′, with thereference length Lr of 800 μm, that substantially equals to that of TireC, the tire gloss of Tire A is insufficient, being 0.4. Further, thetire surface of Tire A has a black and yet coarse texture. Accordingly,Tire A does not have a good appearance.

Tire B has a small arithmetical mean roughness Ra, with the referencelength Lr of 20 μm, that is 0.05. Accordingly, Tire B has an overly hightire gloss and a whitish and shiny texture, and therefore does not havea good appearance.

On the other hand, Tire C, in which the arithmetical mean roughness Ra,with the reference length Lr of 20 μm, was in the range of 0.1 μm to 0.6μm, had an appropriate gloss and a deeply-black and moist texture, andtherefore had a good appearance.

Further, a plurality of tires which had, on their tire surfaces,different arithmetical mean roughness Ra, with the reference length Lrof 20 μm, were manufactured, and then the relationship between thearithmetical mean roughness Ra and the tire gloss was evaluated. Theevaluation results shown in the graph of FIG. 4 were obtained.

According to FIG. 4, by configuring the arithmetical mean roughness Raof the tire surface, with the reference length Lr of 20 μm, to be notmore than 0.6 μm, the gloss of the tire surface may be not less than 1.

Further, a plurality of tires, which had, on their tire surfaces, thearithmetical mean roughness Ra in the range of 0.1 μm to 0.6 μm, withthe reference length Lr of 20 μm, and had different irregularitydiameters and irregularity depths on their tire surfaces, were evaluatedon the condition of their tire surfaces. The evaluation results shown inthe following Table 8 were obtained.

TABLE 8 Conventional Inventive Inventive Example Example Example tire 1tire 1 tire 2 Method of processing the Process A Process B Process Csurface of each mold for respective tire Condition of tire surface Blackand yet Black and Black and (Sensory evaluation) coarse moist moistIrregularity diameter 9-36 40-85 100-169 Irregularity depth 8-18  3-10 9-21 Inventive Comparative Example Example tire 3 tire 1 Method ofprocessing the Process D Process F surface of each mold for respectivetire Condition of tire surface Black and Deepest black (Sensoryevaluation) moist and yet very coarse Irregularity diameter 150-230220-360 Irregularity depth 15-22 18-30

According to Table 8, Conventional Example tire 1 with a smallirregularity diameter and a small irregularity depth had a black and yetcoarse texture on its tire surface, Comparative Example tire 1 with alarge irregularity diameter and a large irregularity depth had adeepest-black and yet very coarse texture on its surface. Therefore,none of the tires had a good appearance.

On the contrary, each of Inventive tires 1-3, which had the irregularitydiameter and irregularity depth that fell under the scope of the presentinvention, had a deeply-black and moist texture, and therefore had agood appearance.

From the foregoing evaluation results and the data of Tables 3, 4 and 5,the ranges of the irregularity diameter and irregularity depth whichprovided a tire surface in a good condition were extracted. The regionshown in FIG. 5 was obtained.

FIG. 5 is a scatter plot showing the mean values of five measurementvalues of the irregularity diameter and irregularity depth,respectively.

The condition of the tire surface was determined either good or bad, byfive panelists through sensory evaluation.

1. A method of manufacturing a tire vulcanization mold, the mold havinga first portion corresponding to a tire tread portion, and having secondand third portions corresponding to tire side portions, each of thefirst, second and third portions having a surface, the methodcomprising: propelling spherical abrasive grains onto at least one ofthe surfaces of the first, second and third portions under the conditionthat a propelling pressure is in the range of 0.1-0.6 MPa, so as toprocess the surface of the mold.
 2. The method of manufacturing a tirevulcanization mold according to claim 1, wherein the abrasive grainswith a grain diameter of not more than 850 μm are propelled onto thesurface of the mold.
 3. The method of manufacturing a tire vulcanizationmold according to claim 1, wherein the abrasive grains are propelledonto the surface of the mold at the propelling pressure of 0.1 MPa to0.2 MPa.
 4. The method of manufacturing a tire vulcanization moldaccording to claim 2, wherein the abrasive grains are propelled onto thesurface of the mold at the propelling pressure of 0.1 MPa to 0.2 MPa.